Recurrent breakdown of Late Permian reef communities in response to episodic volcanic activities: evidence from southern Guizhou in South China

Reefs, both living and ancient, are extremely sensitive to environmental change. Recurrent breakdown of reef communities implies episodic occurrence of unfavorable marine conditions. An alternating succession of reef limestone with algal-foraminiferal grainstone records frequent change of Late Permian shallow-marine ecology in the Ziyun area of Guizhou Province, South China. The algal-foraminiferal grainstone interbedded in the marginal platform reef succession there has long been regarded as back-reef, lagoonal deposits, indicating lateral facies changes as the succession developed. However, our research reveals, for the first time, abundant pristine quartz crystals and volcanic glass scattered in the interbedded algal-foraminiferal layers but not in reef facies, suggesting temporal environmental changes and not a simple facies shift. Many quartz crystals form overgrowths nucleated on smaller quartz crystals; the overgrowths are diagenetic, but the nuclei are good evidence of a volcanic source. Therefore, the alternating formation of reef limestone and algal-foraminiferal limestone is interpreted as the result of episodic volcanic activity during the Late Permian. Temporary punctuations by nearby volcanic eruptions are suggested to have caused recurrent breakdown of reef communities and the occupation of reef ecological space by an algal-foraminiferal fauna. The quartz crystals are evidence that this interpretation is more likely than other controls such as sea-level changes. Cement-rich encrusted framestone (comprised of Archaeolithoporella encrusting sponge) at the top of the reef succession, as well as abundant volcanic quartz, implies that both volcanism and increased temperature may be involved in leading to the complete collapse of the reef ecosystem flourishing in Changhsingian time in South China.